Check with your lawyer. If your Dad is of sound mind, and you don't exert any undo pressure he should be able to give everything to you, unless you have siblings. A new will would render any previous will moot. I hate the idea that the Witnesses can treat somebody like garbage and then expect to collect their estate.
What state are you in. My family's estate planner may know somebody that could help. He knows people all over the place. PM me if you want help.